Compositions comprising phosphoric acid and various insoluble pigments (e.g., aluminum metal) are well known for use in protecting metallic surfaces such as ferrous alloy surfaces from corrosion. In such coating compositions, particulate metallic aluminum such as flake and/or powdered aluminum is combined with a phosphoric acid bonding solution to form a coating composition which is then applied to the metallic surface being treated. After application to the surface, the coating composition may be heated to a first temperature, generally at least about 500.degree. F. (260.degree. C.), until the coating is rendered essentially water insoluble. The coated surface may then be cured at a second temperature, generally above 1000.degree. F. (538.degree. C.) to form the final protective coating.
The cured coating prepared from the combination of particulate metallic aluminum and phosphoric acid bonding solution is termed an "undercoat" or "basecoat". It is often further desirable to provide an extra protective barrier to the metal surface that may provide thermal resistance or simply augment the corrosion protection afforded by the coating formed from the coating composition described above. An extra protective layer applied to the cured undercoat is termed a "topcoat". The topcoat may be formed from a bonding solution similar to that used in the undercoat, but containing little or no particulate metal.
The basecoat or topcoat composition may further contain a pigment which imparts visually aesthetic qualities to the coating. The pigment(s) may also be functional and improve certain properties of the coating such as corrosion resistance and bond strength.
Care must be taken in the preparation of phosphate-based coating compositions containing particulate aluminum metal. The phosphoric acid bonding solution can react with the aluminum. Such reactions are considerably exothermic and can be very violent, causing the metallic aluminum to burn or even explode. These reactions may also result in the conversion of the metallic aluminum into various salts which interfere with the formation of suitable protective coatings. Thus, the reactive stability of a coating composition in the presence of metallic aluminum is of foremost concern if the coating composition is used as a basecoat. If the bonding composition is used as a topcoat, reactive stability in the presence of particulate aluminum metal is not a major concern.
U.S. Pat. No. 3,248,251, to Allen, describes coating compositions consisting essentially of a slurry of solid inorganic particulate material (such as metallic aluminum) in an aqueous acidic bonding solution containing dissolved metal chromate, dichromate or molybdate, and phosphate. Allen discloses that the addition of chromates or molybdates to the acidic bonding solution effectively passivates the solution toward aluminum and inhibits the oxidation of metallic aluminum, allowing particulate aluminum to combine with the bonding solution without the undesirable chemical reaction between the acidic bonding solution and the aluminum. These "Allen" coatings have been, and still are, used to provide high-quality coatings which protect ferrous metal alloy surfaces from oxidation and corrosion, particularly at high temperatures.
As understood herein, "chromate" refers equally to chromate ion, dichromate ion, and hexavalent chromium. Chromate ion also passivates the steel substrate to eliminate undesirable reactions such as acidic attack or pitting on the substrate. It is also known that the inclusion of chromate ion in the coating composition provides a coating having improved corrosion resistance.
However, while chromates have been used successfully to reduce the reactivity of the aluminum in coating compositions and to improve the corrosion resistance of the coatings, the use of chromates has become a problem because of environmental considerations. Chromates are considered to be toxic substances. Hexavalent chromium is a carcinogen. It is therefore desirable to avoid the use of solutions of their salts, or at least to reduce their use. For this reason, it has been desirable to develop a phosphate/aluminum corrosion resistant coating composition which requires little or no chromate or molybdate. Such coating compositions should protect ferrous metal alloy surfaces from oxidation and corrosive environmental conditions, especially at high temperatures, approximately as well as and preferably better than the so-called Allen coatings (i.e., per U.S. Pat. No. 3,248,251).
Efforts have been made to exclude chromate and molybdate from coating compositions while maintaining stable formulations. U.S. Pat. No. 5,242,488 to Stetson et al., describes a basecoat coating composition for ferrous alloys which does not require either chromates or molybdates to control the reaction between the bonding solution and the powdered aluminum. The composition consists essentially of a slurry mixture of a bonding solution and aluminum powder. The bonding solution consists essentially of water, phosphoric acid (H.sub.3 PO.sub.4), and aluminum ions. The bonding solution must contain aluminum ions in solution such that the amount of aluminum in solution is substantially at the saturation point, thus leaving the bonding solution essentially inert with respect to any subsequent additions of aluminum.
U.S. Pat. No. 5,279,649, also to Stetson, et al., discloses substantially the same compositions, but to which V.sub.2 O.sub.5 has been added as a source of vanadate ion, adding another inhibitor to the aluminum equilibrated mixture. Addition of V.sub.2 O.sub.5 is an example of the addition of a toxic substance, listed on the EPA SARA Extremely Hazardous Substance List and also subject to Clean Air Act and CERCLA regulation.
Further, in U.S. Pat. No. 5,279,650, also to Stetson, et al., a seal coating (topcoat) composition containing vanadate ion and iron oxide (Fe.sub.2 O.sub.3) powder is disclosed.
All three of these Stetson coating compositions are designed to avoid the use of chromate and molybdate ions and require the bonding solution to be equilibrated with respect to further additions of aluminum as described in these patents.
Although the Stetson patents indicate that these formulations control the reactivity between the bonding solution and the aluminum, some reaction still occurs between the bonding solution and the powdered aluminum when the slurry compositions of the Stetson patents are formulated.
U.S. Pat. No. 5,478,413 to Mosser et al. is directed to coating compositions lacking chromate or molybdate. These coating compositions are pigmented with metallic aluminum powder and can be applied to all ferrous alloys. These coatings may require a topcoat to be applied thereon for satisfactory protection of the metal substrate in some applications.
It is therefore desired to formulate a chromate- and molybdate-free bonding composition, or one which is of reduced chromate and molybdate content, which not only has a reduced reactivity with particulate aluminum when the two are combined to form a coating composition, but also enables the formulation of an effective coating while being free of toxic additives such as hexavalent chromium.